Not using DFS but...
I have been having similar problems with my 2003 server. Windows take forever to login to domain. System resources / shares are no longer available over time. You cannot login with remote desktop or even at the server. It says Applying Settings forever. You have to crash the server hard! To ensure it was Avast, I installed twice and had the same problems. Initially it seems OK but within a week the problems arise. After the uninstall the server runs as good as it can.
We have another 2003 server running our EMR program that is experiencing unexplained slow downs. Removing the AV product from it tonight or tomorrow. Will advise on that when complete.
The Avast home version is great but I think they have some work to do on their corporate products. I really want to buy their software for 6 servers and 125 desktops but could not recommend them at this time.